A low-cost method for preparing a titanium alloy investment casting shell

Abstract

The application provides a low-cost titanium alloy investment casting shell preparation method, and belongs to the technical field of titanium alloy investment casting, to solve the problem of replacing inert materials with ordinary precision casting materials to prepare low-cost titanium alloy shells. The method comprises the following steps: preparing a surface layer and a back layer on the surface of a wax mold; the surface layer is coated with a coating containing silica sol and zirconium powder and is sprinkled with zirconium sand aggregate; the back layer is coated with a coating containing aluminum sol and mullite powder; the shell is subjected to dewaxing treatment to obtain a dewaxed mold shell; the dewaxed mold shell is placed in a vacuum roasting furnace, and after vacuumizing, low positive pressure hydrogen atmosphere is formed by filling hydrogen, while continuously pumping to remove reaction products, and the temperature is raised to 1100-1300 DEG C and kept; the roasted shell is used for titanium alloy casting. The application realizes the casting effect of traditional inert oxide shells with the cost of ordinary precision casting materials, and the shell cost is reduced to one fifth to one tenth of the original technology, and the surface quality of the casting is basically the same as that of the yttrium oxide shell.

Description

Technical Field

[0001] This application belongs to the field of titanium alloy investment casting technology, specifically relating to a low-cost method for preparing titanium alloy investment casting mold shells. Background Technology

[0002] Titanium alloys, due to their superior properties such as high specific strength, low density, and good corrosion resistance, are widely used in aerospace, shipbuilding, and petrochemical industries. Investment casting, as a primary method for precision forming of titanium alloys, offers technical advantages such as high dimensional accuracy and good formability. However, titanium alloys have a high melting point and are extremely chemically reactive, reacting with most casting materials in their molten state at high temperatures, leading to surface contamination, deterioration of mechanical properties, and even scrapping of the castings. Therefore, the surface material of titanium alloy investment casting molds must be a chemically inert material, such as yttrium oxide, zirconium oxide, or tungsten.

[0003] Currently, the main technical solutions for preparing titanium alloy investment casting shells are as follows. The first is the inert oxide surface layer solution, using yttrium oxide or high-purity zirconium oxide as the surface refractory material, combined with yttrium sol or zirconium sol as a binder. While this solution meets the chemical stability requirements of titanium alloy casting, the raw material costs of yttrium oxide and high-purity zirconium oxide are extremely high, resulting in shell preparation costs that are 5 to 10 times higher than those of ordinary precision casting shells, severely restricting the large-scale industrial application of titanium alloy castings. The second is the ordinary material modification solution. Some existing technologies attempt to coat the surface of ordinary silicon-based shell materials with an isolation layer or use special sintering processes to reduce reactivity, but neither fundamentally solves the problem of the chemical reaction between silica and titanium alloys at high temperatures. Untreated zircon powder and silica sol systems will react violently with molten titanium at the titanium alloy casting temperature, forming a brittle reaction layer, leading to deterioration of the casting surface quality and making it difficult to clean.

[0004] Therefore, there is an urgent need to propose a method for preparing low-cost shells that can replace expensive inert oxide materials with ordinary precision casting materials to meet the requirements of titanium alloy investment casting. Summary of the Invention

[0005] In view of this, this application provides a low-cost titanium alloy investment casting shell preparation method to solve the problem of preparing low-cost titanium alloy shells by replacing inert materials with ordinary precision casting materials.

[0006] To solve the above problems, the technical solution adopted in this application is as follows:

[0007] In the first aspect, this application proposes a low-cost method for preparing titanium alloy investment casting shells, comprising the following steps: Step 1: preparing a surface layer and a back layer on the surface of a wax pattern; the surface layer is coated with a coating containing silica sol and zircon powder and sprinkled with zircon sand aggregate; the back layer is coated with a coating containing aluminum sol and mullite powder; Step 2: dewaxing the shell to obtain a dewaxed shell; Step 3: placing the dewaxed shell in a vacuum calcining furnace, under a low positive pressure hydrogen atmosphere formed by evacuation and hydrogen filling, while continuously evacuating to remove reaction products, heating to 1100-1300℃ and holding at that temperature, causing the silicon-containing material in the surface layer to undergo a reduction reaction to generate gaseous products and be removed, thereby transforming the surface layer in situ into a ceramic layer with ZrO2 as the main phase; Step 4: using the calcined shell for titanium alloy casting.

[0008] Furthermore, in step 3, the furnace is first evacuated to below 1 Pa and then filled with hydrogen gas, maintaining the hydrogen pressure in the furnace at 0.05-0.15 MPa; the temperature is raised to 1100-1200℃ and held for 1-3 hours; then the temperature is lowered to 300℃, and the furnace is evacuated and the vacuum is broken.

[0009] Furthermore, in step 1, the back layer includes a transition layer and a reinforcing layer sequentially disposed on the outside of the surface layer.

[0010] Furthermore, the back layer also includes a sealing layer disposed outside the reinforcing layer, and the transition layer, reinforcing layer and sealing layer are all coated with a coating containing aluminum sol and mullite powder; the number of the surface layer, transition layer, reinforcing layer and sealing layer is adjusted according to the casting size and structural complexity.

[0011] Furthermore, the dewaxing in step 2 is steam dewaxing, with a steam pressure of 0.75±0.05MPa, a temperature of 168±5℃, a pressure of 0.6MPa in 10 seconds, and a dewaxing time of 15-30min.

[0012] Furthermore, the silicon-containing material in step 3 includes SiO2 and ZrSiO4, and the reduction reaction generates gaseous SiO and H2O, which are removed by continuous evacuation; and the Al2O3 and mullite in the back layer do not undergo reduction reaction under the low positive pressure hydrogen atmosphere and continuous evacuation conditions.

[0013] Further, in step 1, the coating temperature of the topcoat is 22±2℃, the humidity is 40-70%, and the drying time is 6-12h; the coating humidity of the backcoat is 30-60%, and the drying time is 12-24h.

[0014] Secondly, this application also proposes a coating system for the low-cost titanium alloy investment casting shell preparation method described in the first aspect, comprising a top coating and a back coating, wherein: the top coating comprises silica sol and zircon powder, used to undergo a reduction reaction under hydrogen vacuum calcination conditions and be converted in situ into a ceramic layer with ZrO2 as the main phase; the back coating comprises alumina sol and mullite powder, used to maintain structural stability under the hydrogen vacuum calcination conditions.

[0015] Further, the topcoat coating comprises, by weight, 10 parts silica sol, 38-42 parts zircon powder, 16-20 ml wetting agent, and 12-16 ml defoamer, with a flow cup viscosity of 22-32 s; the backcoat coating comprises a transition layer coating and a reinforcing layer coating, the transition layer coating comprising, by weight, 10 parts alumina sol and 15-20 parts mullite powder, with a flow cup viscosity of 12-16 s; the reinforcing layer coating comprising, by weight, 10 parts alumina sol and 28-32 parts mullite powder, with a flow cup viscosity of 23-28 s; and a sealing coating comprising, by weight, 10 parts alumina sol and 10-18 parts mullite powder, with a flow cup viscosity of 7-15 s.

[0016] Thirdly, this application also proposes a titanium alloy investment casting shell having a face layer and a back layer. The face layer is a ceramic layer formed by vacuum calcination and reduction with hydrogen, having ZrO2 as the main crystalline phase and a ZrO2 mass fraction of 80-95%. The back layer is a ceramic support layer composed of alumina sol as a binder and mullite as a refractory aggregate.

[0017] Furthermore, the surface layer contains less than 5% SiO2 by mass and less than 2wt% Si by element; the back layer contains 30-50% Al2O3 by mass.

[0018] In summary, due to the adoption of the above technical solution, the beneficial effects of this application are:

[0019] The surface layer of this application is coated with a paint containing silica sol and zircon powder and sprinkled with zircon sand aggregate. This material system is considered reactive and excluded in conventional titanium alloy casting. However, this application combines it with a hydrogen vacuum calcination step, causing the silica-containing material in the surface layer to undergo a reduction reaction under the action of hydrogen, generating gaseous products that are then removed, thereby transforming it in situ into a ceramic layer with zirconium oxide as the main phase. The back layer is coated with a paint containing aluminum sol and mullite powder. This material system does not undergo a reduction reaction under hydrogen atmosphere and high temperature conditions, maintaining structural stability. If the back layer also uses a silica-containing binder, hydrogen calcination would cause the back layer binder to be reduced and removed, and the entire shell would collapse due to loss of structural support. Therefore, the reducibility of the surface layer and the non-reducibility of the back layer form functional zones, both of which are indispensable and together constitute the heterogeneous layered structure of the hydrogen vacuum calcined shell.

[0020] The dewaxing step occurs between shell formation and hydrogen vacuum roasting. If dewaxing is incomplete, the residual wax will carbonize or volatilize during the subsequent high-temperature hydrogen roasting, contaminating the furnace atmosphere and hindering sufficient contact between hydrogen and the surface material, thus affecting the uniformity and completeness of the reduction reaction. Simultaneously, the initial porous structure of the shell after dewaxing provides a pathway for the subsequent discharge of gaseous reaction products.

[0021] In the hydrogen vacuum calcination step, after evacuation, hydrogen is introduced to form a low-positive-pressure hydrogen atmosphere, while continuous evacuation removes reaction products. This low-positive-pressure hydrogen atmosphere ensures sufficient hydrogen concentration to drive the reduction reaction and maintains the thermodynamic driving force for the conversion of reactants into gaseous products. The simultaneous operation of continuous evacuation and the low-positive-pressure hydrogen atmosphere creates a dynamic equilibrium: continuous hydrogen replenishment maintains the reducing agent concentration, while vacuum evacuation promptly removes generated silica and water vapor, reducing the partial pressure of gaseous products and preventing reverse reaction or product recondensation and deposition in the shell pores during the cooling phase. Heating to 1100-1300℃ and holding at this temperature ensures the thermodynamic feasibility of the reduction reaction of silica and zirconium silicate, while alumina and mullite remain stable, achieving simultaneous surface modification and back-layer reinforcement.

[0022] The casting process utilizes the inert shell constructed in the preceding steps for practical application. After hydrogen vacuum calcination, the surface layer transforms into a ceramic layer with zirconium oxide as the main phase. This layer exhibits significantly reduced reactivity with the molten titanium alloy, meeting the surface quality requirements for precision casting of titanium alloys. The back layer maintains its original structural strength, withstanding the hydrostatic pressure and thermal stress of the molten metal, ensuring the dimensional accuracy of the casting.

[0023] This application achieves the casting effect of traditional inert oxide shells at the cost of ordinary precision casting materials, reducing shell costs to one-fifth to one-tenth of the original technology. Simultaneously, the surface quality of the castings is essentially the same as that of yttrium oxide shells, far superior to the severely reactive state of untreated ordinary shells. Each step is interconnected, and material selection and process conditions are mutually compatible. The loss or replacement of any single feature will lead to the loss or deterioration of the overall technical effect. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein:

[0025] Figure 1 This is a schematic flowchart illustrating the low-cost titanium alloy investment casting shell preparation method provided in the embodiments of this application. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. The components of the embodiments of this application described and shown in the accompanying drawings can be arranged and designed in various different configurations.

[0027] Therefore, the following detailed description of the embodiments of this application provided in the accompanying drawings is not intended to limit the scope of the claimed application, but merely to illustrate selected embodiments of the application. All other embodiments obtained by those skilled in the art based on the embodiments in this application without inventive effort are within the scope of protection of this application.

[0028] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other.

[0029] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0030] In this application, unless otherwise expressly specified and limited, "above" or "below" a second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of a second feature includes the first feature directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" a second feature includes the first feature directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature. The terms "first," "second," etc., used in the specification and claims of this application are used to distinguish similar objects and are not used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that embodiments of this application can be implemented in orders other than those illustrated or described herein, and the objects distinguished by "first," "second," etc., are generally of the same class and do not limit the number of objects; for example, a first object can be one or more. Furthermore, "and / or" in the specification and claims indicates at least one of the connected objects, and the character " / " generally indicates that the preceding and following objects are in an "or" relationship.

[0031] Titanium alloys, due to their superior properties such as high specific strength, low density, and good corrosion resistance, are widely used in aerospace, shipbuilding, and petrochemical industries. Investment casting, as a primary method for precision forming of titanium alloys, offers advantages such as high dimensional accuracy and good formability. However, titanium alloys have high melting points and are extremely chemically reactive, reacting with most casting materials in their molten state at high temperatures, leading to surface contamination, deterioration of mechanical properties, and even scrapping of the castings. Therefore, the surface layer material for titanium alloy investment casting shells must be a chemically inert material, such as yttrium oxide, zirconium oxide, or tungsten. Currently, the preparation of titanium alloy investment casting shells mainly employs the following technical solutions. The first is the inert oxide surface layer solution, using yttrium oxide or high-purity zirconium oxide as the surface refractory material, combined with yttrium sol or zirconium sol as a binder. While this approach meets the chemical stability requirements of titanium alloy casting, the extremely high cost of yttrium oxide and high-purity zirconium oxide makes the cost of mold shell preparation 5 to 10 times that of ordinary precision casting mold shells, severely restricting the large-scale industrial application of titanium alloy castings. The second approach involves modifying ordinary materials. Some existing technologies attempt to coat the surface of ordinary silicon-based mold shell materials with an isolation layer or employ special sintering processes to reduce reactivity, but none of these fundamentally solve the problem of the chemical reaction between silica and titanium alloys at high temperatures. Untreated zircon powder and silica sol systems react violently with molten titanium at the titanium alloy casting temperature, forming a brittle reaction layer that deteriorates the surface quality of the casting and is difficult to clean.

[0032] This application achieves the casting effect of traditional inert oxide shells at the cost of ordinary precision casting materials, reducing shell costs to one-fifth to one-tenth of the original technology. Simultaneously, the surface quality of the castings is essentially the same as that of yttrium oxide shells, far superior to the severely reactive state of untreated ordinary shells. Each step is interconnected, and material selection and process conditions are mutually compatible. The loss or replacement of any single feature will lead to the loss or deterioration of the overall technical effect.

[0033] The following is in conjunction with the appendix Figure 1 The technical solutions provided in this application will be described in detail through specific embodiments and application scenarios.

[0034] In the first aspect, this application proposes a low-cost method for preparing titanium alloy investment casting shells, comprising the following steps: Step 1: preparing a surface layer and a back layer on the surface of a wax pattern; the surface layer is coated with a coating containing silica sol and zircon powder and sprinkled with zircon sand aggregate; the back layer is coated with a coating containing aluminum sol and mullite powder; Step 2: dewaxing the shell to obtain a dewaxed shell; Step 3: placing the dewaxed shell in a vacuum calcining furnace, under a low positive pressure hydrogen atmosphere formed by evacuation and hydrogen filling, while continuously evacuating to remove reaction products, heating to 1100-1300℃ and holding at that temperature, causing the silicon-containing material in the surface layer to undergo a reduction reaction to generate gaseous products and be removed, thereby transforming the surface layer in situ into a ceramic layer with ZrO2 as the main phase; Step 4: using the calcined shell for titanium alloy casting.

[0035] Specifically, both the silica in the silica sol and the zircon in the zircon powder contain silicon-oxygen bonds. Under high-temperature reduction conditions with hydrogen, they can decompose to generate gaseous silica and water vapor, while the zircon element is transformed into stable zirconium oxide and remains in the surface layer. This transformation process achieves a fundamental shift from a reactive material to an inert material, enabling the ordinary zircon system, which was originally unsuitable for titanium alloy casting, to acquire chemical stability essentially the same as that of expensive yttrium oxide shells.

[0036] The backing layer uses a coating containing aluminum sol and mullite powder, forming a clear functional zone with the surface layer's material system. The alumina and mullite in the aluminum sol do not undergo reduction reactions in a hydrogen atmosphere at 1100-1300℃, a crucial characteristic. If the backing layer also used a silica sol binder, hydrogen roasting would destroy the entire shell's binder system, causing the shell to crumble and collapse due to loss of structural support. The aluminum sol backing layer maintains its stability during the reduction reaction of the surface layer, thus providing continuous mechanical support to the shell and ensuring the overall structural integrity of the shell while the surface layer completes its inertization transformation.

[0037] The dewaxing step occurs between shell formation and hydrogen vacuum roasting. Steam dewaxing rapidly removes the wax mold at a pressure of approximately 0.75 MPa and a temperature of 168°C, forming a hollow shell. If dewaxing is incomplete, residual wax will carbonize or volatilize during subsequent high-temperature hydrogen roasting, contaminating the furnace atmosphere and hindering sufficient contact between hydrogen and the surface material, affecting the uniformity and completeness of the reduction reaction. Simultaneously, the initial porous structure of the shell after dewaxing provides a pathway for the subsequent discharge of gaseous reaction products.

[0038] Hydrogen vacuum calcination is the core step of the entire method. After evacuation, hydrogen is introduced to create a low-positive-pressure hydrogen atmosphere, distinct from atmospheric pressure hydrogen or pure vacuum environments: atmospheric pressure hydrogen has a higher partial pressure of reaction products, which is unfavorable for the forward reduction reaction; pure vacuum lacks the reducing agent hydrogen. The low-positive-pressure hydrogen atmosphere ensures sufficient hydrogen concentration to drive the reduction reaction and maintains the thermodynamic driving force for the conversion of reactants into gaseous products. Continuous evacuation and the low-positive-pressure hydrogen atmosphere operate synchronously, forming a dynamic equilibrium: continuous hydrogen replenishment maintains the reducing agent concentration required for the reaction, while vacuum evacuation promptly removes generated silica and water vapor, reducing the partial pressure of gaseous products and preventing the reaction from proceeding in reverse or the products from recondensing and depositing in the shell pores during the cooling phase. This dynamic removal mechanism is crucial for achieving complete inertization of the surface layer. If gaseous products remain, secondary deposition will occur inside the shell, resulting in a high residual silicon content in the surface layer and incomplete reduction of reactivity with the titanium alloy. The temperature was raised to 1100-1300℃ and held. This temperature range was thermodynamically optimized: if the temperature is too low, the reduction reaction rate will be insufficient or the reaction will be incomplete; if the temperature is too high, the shell may soften and deform. At this temperature, the reduction reaction of silica and zirconium silicate is thermodynamically feasible, while alumina and mullite remain stable, achieving simultaneous surface layer modification and back layer reinforcement.

[0039] After being calcined in a hydrogen vacuum, the surface layer is transformed into a ceramic layer with zirconium oxide as the main phase. This layer exhibits significantly reduced reactivity with the molten titanium alloy, thus meeting the surface quality requirements for precision casting of titanium alloys. The back layer maintains its original structural strength, withstands the hydrostatic pressure and thermal stress of the molten metal, and ensures the dimensional accuracy of the casting.

[0040] In summary, the casting effect of traditional inert oxide shells was achieved at the cost of ordinary precision casting materials, effectively reducing shell costs. Simultaneously, the surface quality of the castings is essentially the same as that of yttrium oxide shells, far superior to the severely reactive state of untreated ordinary shells. Each step is interconnected, and material selection and process conditions are mutually compatible. The loss or replacement of any single feature will lead to the loss or deterioration of the overall technical effect.

[0041] In some embodiments, in step 3, the furnace is first evacuated to below 1 Pa and then filled with hydrogen gas, maintaining the hydrogen pressure in the furnace at 0.05-0.15 MPa; the temperature is raised to 1100-1200℃ and held for 1-3 hours; then the temperature is lowered to 300℃, and the furnace is evacuated and the air is broken.

[0042] After evacuating to below 1 Pa, hydrogen is introduced. This vacuum level is required to ensure that residual air in the furnace is fully removed, preventing the formation of an explosive mixture of oxygen and hydrogen, and eliminating the oxidation interference of oxygen in the air on the surface material. If the vacuum level is insufficient, residual oxygen will react with zircon powder at high temperatures to form unintended oxides, hindering the forward reduction reaction and even causing oxidation and discoloration of the mold shell surface. Maintaining the hydrogen pressure in the furnace at 0.05-0.15 MPa creates a low positive pressure hydrogen atmosphere. This pressure range balances safety and reaction efficiency: below 0.05 MPa, the hydrogen concentration is too low, resulting in insufficient driving force for the reduction reaction, leading to a slow and incomplete reaction rate; above 0.15 MPa, hydrogen consumption increases, safety risks rise, and the marginal effect on improving the reaction rate diminishes. The temperature is raised to 1100-1200℃ and held for 1-3 hours. This temperature range is lower than the theoretical reduction temperature of pure silicon dioxide, but because the actual silicon dioxide in the surface layer exists in an amorphous or colloidal state, its reactivity is higher than that of crystalline quartz. Therefore, effective reduction can be achieved at 1100-1200℃. Holding the temperature for 1-3 hours ensures that the heat is fully transferred to the core of the mold shell, so that the entire surface layer completes the reduction transformation uniformly, rather than just the surface thin layer. Then, the temperature is lowered to 300℃, and a vacuum is drawn and the vacuum is broken. This operation sequence has a specific safety logic: 300℃ is lower than the auto-ignition temperature of hydrogen. At this time, drawing a vacuum can further remove residual hydrogen and reduce the risk of introducing air during the vacuum breaking to form an explosive mixture; if the vacuum is broken directly at the high temperature stage, the influx of air may react with the high-temperature hydrogen and cause a safety accident.

[0043] In some embodiments, in step 1, the back layer includes a transition layer and a reinforcing layer sequentially disposed outside the surface layer; the back layer also includes a sealing layer disposed outside the reinforcing layer; the transition layer, the reinforcing layer and the sealing layer are all coated with a coating containing aluminum sol and mullite powder; the number of the surface layer, the transition layer, the reinforcing layer and the sealing layer is adjusted according to the casting size and structural complexity.

[0044] The back layer adopts a layered structure design, including a transition layer and a reinforcing layer sequentially located outside the surface layer, and a sealing layer located outside the reinforcing layer. All three layers are coated with a paint containing aluminum sol and mullite powder. The transition layer is adjacent to the surface layer and serves as the interface connector. Its paint penetrates into the gaps between the aggregates of the surface layer, forming a dense transition interface, effectively mitigating the thermal expansion mismatch caused by the material difference between the surface layer and the reinforcing layer, and preventing interlayer delamination during high-temperature firing and metal casting. The reinforcing layer is located outside the transition layer and is the main supporting part of the shell structure strength. It forms a load-bearing skeleton through multi-layer stacking, resisting the hydrostatic pressure and thermal stress impact of molten metal. The sealing layer is located on the outermost side of the shell. It uses the low viscosity characteristics of the paint to fill the surface pores of the reinforcing layer, forming a smooth and dense outer surface. This prevents the shell from losing sand and loosening during handling, firing, and casting, while also blocking external impurities from entering the interior of the shell.

[0045] The transition layer, reinforcing layer, and sealing layer all utilize a coating system containing aluminum sol and mullite powder. This material selection is highly compatible with the hydrogen vacuum calcination process. The alumina and mullite in the aluminum sol do not undergo reduction reactions under hydrogen atmosphere and high-temperature conditions, ensuring the chemical stability and mechanical integrity of the entire back layer structure during the reduction and removal of silicon-containing materials from the surface layer by hydrogen. If a silicon-containing binder were used for the back layer, hydrogen calcination would cause a reduction reaction in the back layer as well, leading to the loss of structural support and collapse of the entire shell. Therefore, the functional zoning design of the surface layer material is a prerequisite for achieving overall shell inertness.

[0046] The number of surface layer, transition layer, reinforcing layer, and sealing layer is adjusted according to the casting size and structural complexity. This adaptive configuration mechanism synergizes with the shell strength requirements and the hydrogen reduction process. Small-sized castings have a small volume of molten metal and low heat load, so fewer layers are sufficient to meet strength requirements, while reducing paint consumption and drying time, thus lowering production costs. Large-sized castings have high static pressure and high thermal stress from the molten metal; increasing the number of reinforcing layers enhances load-bearing capacity, ensuring the shell does not crack or deform during filling. Complex castings have thin walls, sharp corners, or deep cavities where stress is concentrated; increasing the thickness of the surface layer or transition layer improves local strength. During layer adjustment, the surface layer maintains a standard configuration that matches the hydrogen reduction process, ensuring consistent inertization regardless of casting size. The flexibility of the layer configuration allows this method to cover titanium alloy castings ranging in size from tens of millimeters to 1300 millimeters, achieving a balance between process standardization and product structuring.

[0047] The number of layers can be adjusted to cover a size range from tens of millimeters to 1,300 millimeters. Precise matching of shell strength to casting requirements is achieved through variations in the combination of surface layers, transition layers, reinforcing layers, and sealing layers. For small castings less than 50 millimeters in size, a configuration of one surface layer, one transition layer, and two reinforcing layers provides sufficient strength at both room and high temperatures. Due to the small casting size and low volume of molten metal, the shell experiences lower thermal and mechanical loads, eliminating the need for a sealing layer to meet casting requirements. For small castings less than 100 millimeters in size, the surface layer is increased to two layers, while the remaining layers consist of one transition layer and two reinforcing layers. The increased surface layer thickness enhances the thickness of the inert protective layer in the contact area between the inner surface of the shell and the molten titanium alloy. Simultaneously, the number of transition and reinforcing layers remains unchanged to ensure the backing structure does not become excessively thickened, maintaining good permeability.

[0048] For small castings less than 200 mm in size, a configuration of two surface layers, one transition layer, two reinforcing layers, and an additional sealing layer is used. The sealing layer seals the pores on the surface of the reinforcing layer, preventing sand from falling off the mold shell during handling and high-temperature treatment. It also provides a smoother outer surface for small, complex castings, reducing the intrusion of external impurities. For small to medium-sized parts less than 400 mm in size, the reinforcing layer is increased to four layers, while the sealing layer is retained. This configuration significantly improves the mold shell's ability to withstand the hydrostatic pressure of the molten metal. The four reinforcing layers form a thicker skeletal structure, resisting the impact of the molten metal during filling and the stress of solidification shrinkage.

[0049] For large castings smaller than 800 mm, the reinforcement layer is further increased to six layers, combined with two surface layers, a transition layer, and a sealing layer. This structure provides sufficient overall rigidity and high-temperature strength for large castings, preventing deformation or cracking due to the weight of the shell and the pressure of the molten metal. For ultra-large castings between 800 mm and 1300 mm, the reinforcement layer is increased to eight layers, forming the thickest back support system. This configuration ensures that the shell maintains structural integrity even with ultra-long spans, while the two surface layers always maintain a standard thickness matching the hydrogen reduction process, ensuring consistent surface inertization conversion regardless of the casting size.

[0050] In all the above configurations, the surface layer is consistently maintained at a standard configuration of one to two layers to ensure uniform reduction reaction during hydrogen vacuum calcination, preventing incomplete reduction or excessive residual silicon content in the inner layer due to an excessively thick surface layer. The transition layer is always a single layer, serving as a fixed buffer interface between the surface layer and the reinforcing layer, and its function remains unchanged regardless of the number of reinforcing layers. The number of reinforcing layers increases from two to eight, constituting the main regulating variable for shell strength and directly responding to the increased structural load caused by the increase in casting size. A sealing layer is introduced in configurations for small and medium-sized parts and above, serving as a sealing layer on the outer surface of the shell; its presence or absence depends on whether the casting size exceeds a certain threshold and the complexity of the structure. This layer configuration system enables shell preparation to possess both a standardized process core and flexible adaptability to product structure.

[0051] In some embodiments, the dewaxing in step 2 is steam dewaxing, with a steam pressure of 0.75±0.05MPa, a temperature of 168±5℃, a pressure of 0.6MPa in 10 seconds, and a dewaxing time of 15-30min.

[0052] A steam pressure of 0.75 MPa corresponds to a saturated steam temperature of approximately 168°C. This temperature is higher than the melting point of commonly used waxes but lower than their decomposition temperature, allowing the wax to melt and flow out quickly rather than carbonizing and leaving residue. The time to reach 0.6 MPa is controlled within 10 seconds to achieve rapid pressure increase and prevent cracking of the mold shell due to uneven expansion of the wax during slow heating. Dewaxing time is 15-30 minutes, differentiated between small and large parts: small parts have less wax and thinner mold walls, requiring 15-20 minutes for complete dewaxing; large parts have more wax and thicker mold walls, requiring 20-30 minutes to ensure heat transfer to the core of the mold shell for complete melting of the internal wax. Dewaxing parameters are directly related to subsequent hydrogen vacuum roasting: incomplete dewaxing results in residual wax carbonizing at temperatures above 1100°C. The carbides react with hydrogen to produce gases such as methane, polluting the furnace atmosphere, consuming hydrogen, and reducing the surface layer reduction efficiency.

[0053] In some embodiments, the silicon-containing material in step 3 includes SiO2 and ZrSiO4, the reduction reaction generates gaseous SiO and H2O which are continuously removed by evacuation; and the Al2O3 and mullite in the back layer do not undergo reduction reaction under the low positive pressure hydrogen atmosphere and continuous evacuation conditions.

[0054] The silicon-containing materials include silicon dioxide and zirconium silicate, whose reaction pathways and products differ during hydrogen reduction. Silicon dioxide is directly reduced to silicon monoxide and water, a relatively simple reaction; zirconium silicate first decomposes into zirconium oxide and silicon dioxide, with the silicon dioxide further reduced, ultimately leaving zirconium oxide. The synergistic presence of these two silicon-containing materials ensures sufficient zirconium oxide formation in the surface layer, forming a continuous and dense zirconium oxide protective layer. The reduction reaction produces gaseous silicon monoxide and water, which are continuously removed by evacuation. The composition of these products determines the design requirements of the vacuum evacuation system: silicon monoxide easily recondenses into solid silicon dioxide when cooled below 800°C, so it must be removed promptly in the high-temperature zone; if water vapor remains, it will react with residual alumina sol in the shell, affecting the stability of the back layer. Alumina and mullite in the back layer do not undergo reduction reactions under low positive pressure hydrogen atmosphere and continuous evacuation conditions. This stability stems from the fact that the standard free energy of formation of alumina and mullite is higher than that of silicon dioxide and zirconium silicate in the stated temperature range, making them thermodynamically less susceptible to hydrogen reduction. This selective reduction characteristic is the fundamental guarantee that the backing layer can maintain structural strength. If the backing layer material can also be reduced, the entire shell will lose its skeletal support and collapse during the firing process.

[0055] In some embodiments, in step 1, the coating temperature of the topcoat is 22±2℃, the humidity is 40-70%, and the drying time is 6-12h; the coating humidity of the backcoat is 30-60%, and the drying time is 12-24h.

[0056] The optimal gelation temperature for silica sol is around 22℃. Too low a temperature results in slow gelation and low production efficiency, while too high a temperature leads to surface crusting and trapped internal moisture, causing blistering. A humidity level of 40-70% controls the moisture evaporation rate. Too low a humidity level causes the surface layer to dry rapidly, forming a hard shell that prevents internal moisture from escaping, leading to surface layer peeling due to steam pressure during subsequent heating. Too high a humidity level results in drying times exceeding 12 hours, and the constantly damp surface layer is prone to microbial contamination. A drying time of 6-12 hours ensures the silica sol is fully dehydrated, forming a three-dimensional network of silicon-oxygen bonds, giving the surface layer initial strength to withstand subsequent back coating and dewaxing operations. The back coating is applied with a humidity level of 30-60% and a drying time of 12-24 hours. The lower upper limit of humidity accommodates the relatively fast drying characteristics of aluminosilicate, preventing the aluminosilicate coating from failing to gel for extended periods due to high ambient humidity, and causing powder sedimentation and stratification. The longer drying time accommodates the characteristic that a single back coating layer is thicker than the surface layer, ensuring complete evaporation of moisture from the core of the thick coating and preventing residual moisture from rapidly vaporizing during subsequent high-temperature stages, which could lead to shell cracking.

[0057] Secondly, this application also proposes a coating system for the low-cost titanium alloy investment casting shell preparation method described in the first aspect, comprising a top coating and a back coating, wherein: the top coating comprises silica sol and zircon powder, used to undergo a reduction reaction under hydrogen vacuum calcination conditions and be converted in situ into a ceramic layer with ZrO2 as the main phase; the back coating comprises alumina sol and mullite powder, used to maintain structural stability under the hydrogen vacuum calcination conditions.

[0058] This coating system is not a simple combination of ordinary precision casting coatings, but a heterogeneous layered system specifically designed for the hydrogen vacuum calcination process. The silica sol in the topcoat provides silica, and the zircon powder provides zirconium silicate. Both undergo reduction reactions under high-temperature hydrogen conditions to generate gaseous products, which are then removed, leaving only zirconium oxide to form an inert ceramic layer. This reducible property allows the topcoat to overcome the technical limitation of traditional precision casting materials being unsuitable for titanium alloy shells. The alumina sol in the backcoat provides alumina as a binder phase, and the mullite powder provides refractory aggregate. Neither undergoes reduction reactions under hydrogen atmosphere and high-temperature conditions, ensuring that while the topcoat undergoes chemical transformation, the backcoat maintains its original structural strength and chemical stability. If the backcoat also used a silica-containing system, hydrogen calcination would cause the backcoat binder to be reduced and removed, leading to the entire shell collapsing due to loss of structural support. Therefore, the reducibility of the topcoat and the non-reducibility of the backcoat form precise functional divisions; both are indispensable and together constitute a specialized coating system for hydrogen vacuum calcination shells.

[0059] In some embodiments, the topcoat coating comprises, by weight, 10 parts silica sol, 38-42 parts zircon powder, 16-20 ml wetting agent, and 12-16 ml defoamer, with a flow cup viscosity of 22-32 s; the backcoat coating comprises a transition layer coating and a reinforcing layer coating, the transition layer coating comprising, by weight, 10 parts alumina sol and 15-20 parts mullite powder, with a flow cup viscosity of 12-16 s; the reinforcing layer coating comprising, by weight, 10 parts alumina sol and 28-32 parts mullite powder, with a flow cup viscosity of 23-28 s; and a sealing coating comprising, by weight, 10 parts alumina sol and 10-18 parts mullite powder, with a flow cup viscosity of 7-15 s.

[0060] The ratio of 10 parts silica sol to 38-42 parts zircon powder in the topcoat ensures sufficient binder content to uniformly suspend and adhere the zircon powder to the wax mold surface. Simultaneously, it ensures a moderate total amount of reducible silicon in the topcoat, meeting the requirements for subsequent hydrogen reduction to generate sufficient zirconium oxide while avoiding excessive silicon content that could lead to incomplete reduction or excessive gaseous products affecting the shell's pore structure. 16-20 ml of wetting agent reduces the surface tension of the slurry, allowing the coating to spread evenly on the complex curved surfaces of the wax mold and preventing whitening at grooves or sharp corners due to surface tension contraction. A dosage below 16 ml results in insufficient wetting, while a dosage above 20 ml leads to excessive surfactant residue affecting the sintering quality of the topcoat. 12-16 ml of defoamer eliminates air bubbles trapped during slurry mixing and coating. The pinholes formed after bubble bursting become channels for the titanium alloy melt to penetrate, leading to sand adhesion defects on the casting surface. The flow cup viscosity of 22-32s comprehensively reflects the consistency of the slurry. This range ensures that the slurry can adhere to the surface of the wax model without flowing, and can also flow into the fine texture to replicate the details of the wax model, providing a uniform surface base for subsequent hydrogen reduction.

[0061] The transition layer coating consists of 10 parts aluminum sol, 15-20 parts mullite powder, and a flow cup viscosity of 12-16s. Its low powder content and low viscosity endow the coating with excellent permeability and flowability, allowing it to fully penetrate into the gaps between the surface layer aggregates to form a dense transition interface, increasing the interlayer contact area and mitigating thermal expansion mismatch caused by material differences between the surface layer and the reinforcement layer. The reinforcement layer coating consists of 10 parts aluminum sol, 28-32 parts mullite powder, and a flow cup viscosity of 23-28s. Its high powder content results in a high coating packing density and a dense skeleton, which is the main source of the shell's room temperature and high temperature strength. The flow cup viscosity is close to the upper limit of the surface layer, balancing the coating's workability and packing thickness, avoiding excessive thinness leading to flow and excessive thickness leading to uneven coating. The sealing coating contains 10 parts aluminum sol, 10-18 parts mullite powder, and a flow cup viscosity of 7-15s. Its minimum powder content and minimum viscosity give the coating excellent leveling properties, which can fill the pores and depressions on the surface of the reinforcement layer, forming a smooth and dense outer surface. This prevents the shell from falling off sand and becoming loose during handling and firing, while also reducing the intrusion of external impurities into the shell.

[0062] The three-layer back coating creates a gradually increasing structural support gradient from the outside to the inside, complementing the inertization function of the topcoat. The transition layer focuses on interfacial bonding, the reinforcement layer on structural load-bearing, and the sealing layer on surface sealing. All three utilize an aluminum sol and mullite powder system to ensure consistent chemical behavior during hydrogen vacuum calcination, maintaining stability and preventing reduction. This creates distinct functional zones compared to the reducible properties of the topcoat. This coating system is deeply integrated with the hydrogen vacuum calcination process described in the first aspect; using it alone does not achieve the same technical effect. Only under hydrogen reduction conditions can the reducibility of the topcoat and the stability of the backcoat work synergistically, enabling the transformation of low-cost materials into high-performance titanium alloy shells.

[0063] Thirdly, this application also proposes a titanium alloy investment casting shell having a face layer and a back layer. The face layer is a ceramic layer formed by vacuum calcination and reduction with hydrogen, having ZrO2 as the main crystalline phase and a ZrO2 mass fraction of 80-95%. The back layer is a ceramic support layer composed of alumina sol as a binder and mullite as a refractory aggregate.

[0064] In some embodiments, the surface layer has a SiO2 mass fraction of less than 5% and a Si element content of less than 2wt%; the back layer has an Al2O3 mass fraction of 30-50%.

[0065] A SiO2 mass fraction of less than 5% and a Si element content of less than 2wt% in the surface layer indicate a sufficient hydrogen reduction reaction, extremely low residual silicon content, and significantly reduced reactivity with the molten titanium alloy, meeting the surface quality requirements for precision casting of titanium alloys. If the SiO2 mass fraction is higher than 5% or the Si element content is higher than 2wt%, the residual silicon content in the surface layer is too high, and a brittle layer will still form upon contact with the titanium alloy, leading to surface contamination and deterioration of mechanical properties in the casting. An Al2O3 mass fraction of 30-50% in the back layer indicates that the alumina sol binder forms a stable alumina bonding phase in the back layer, and the mullite aggregate is uniformly dispersed in the alumina matrix, forming a dense ceramic support structure. This mass fraction range ensures sufficient room temperature and high temperature strength in the back layer. If the Al2O3 content is lower than 30%, the bonding is insufficient and the aggregate is loose; if it is higher than 50%, the aggregate proportion is relatively reduced, and the high-temperature creep resistance decreases. The phase parameters of the surface layer and the back layer corroborate each other, indicating that the shell has undergone a complete hydrogen vacuum calcination process, and the synergistic effect of surface layer inertization and back layer structural stability has reached the industrial application standard.

[0066] Example

[0067] Taking a 150mm titanium alloy casting as an example, the following description illustrates a shell configuration consisting of two surface layers, one transition layer, two reinforcing layers, and one sealing layer. The coatings are prepared according to the following proportions: The surface layer coating, by weight, comprises 10kg silica sol, 38-42kg zircon powder, 16-20ml wetting agent, and 12-16ml defoamer, adjusted to a flow cup viscosity of 22-32s. The transition layer coating, by weight, comprises 10kg alumina sol and 15-20kg mullite powder, adjusted to a flow cup viscosity of 12-16s. The reinforcing layer coating, by weight, comprises 10kg alumina sol and 28-32kg mullite powder, adjusted to a flow cup viscosity of 23-28s. The sealing layer coating, by weight, comprises 10kg alumina sol and 10-18kg mullite powder, adjusted to a flow cup viscosity of 7-15s.

[0068] After cleaning, the wax model is first coated with a topcoat. The topcoat is applied at a temperature of 22±2℃ and a humidity of 40-70%, with zircon sand aggregate sprinkled on top, and dried for 6-12 hours. Subsequently, a transition layer, a reinforcing layer, and a sealing layer are applied in sequence. Each layer of the backcoat is applied at a humidity of 30-60%, and the drying time for the transition layer, reinforcing layer, and sealing layer is 12-24 hours, respectively.

[0069] After the shell is dried, steam dewaxing is performed. The dewaxing steam pressure is 0.75±0.05MPa, the temperature is 168±5℃, the time to reach 0.6MPa pressure is 10 seconds, and the dewaxing time is 15-30 minutes, resulting in a dewaxed shell.

[0070] The dewaxed mold shell is placed in a vacuum calcining furnace, evacuated to below 1 Pa, heated to 1000℃, and filled with hydrogen gas to a pressure of 0.09 MPa. The temperature is then further increased to 1150℃ and held for 1-3 hours. During this holding period, hydrogen gas is continuously supplied while evacuation is performed simultaneously, causing the SiO2 and ZrSiO4 in the surface layer to undergo a reduction reaction, generating gaseous SiO and H2O, which are then promptly removed. This transforms the surface layer in situ into a ceramic layer with ZrO2 as the main phase. Under these conditions, the Al2O3 and mullite in the back layer do not undergo a reduction reaction, maintaining structural stability. After the holding period, the temperature is lowered to 300℃, the vacuum is removed, and the mold shell is taken out for titanium alloy casting.

[0071] Depending on the size and structural complexity of the casting, the number of shell layers can be adjusted as follows: For small parts less than 50mm in size, a configuration of 1 surface layer, 1 transition layer, and 2 reinforcing layers is used. After firing, the shell's room temperature flexural strength is greater than 3.5MPa, its high temperature flexural strength is 4.5MPa, and its residual flexural strength is 2.0MPa. For small parts less than 100mm in size, a configuration of 2 surface layers, 1 transition layer, and 2 reinforcing layers is used. Its room temperature flexural strength is 3.2MPa, its high temperature flexural strength is 4.1MPa, and its residual flexural strength is 1.8MPa. For small parts less than 200mm in size, a configuration of 2 surface layers, 1 transition layer, 2 reinforcing layers, and 1 sealing layer is used. Its room temperature flexural strength is greater than 3.8MPa, its high temperature flexural strength is 5.5MPa, and its residual flexural strength is 2.2MPa. For small to medium-sized parts with dimensions less than 400mm, a configuration of 2 surface layers, 1 transition layer, 4 reinforcing layers, and 1 sealing layer is used, resulting in a room temperature flexural strength of 3.7MPa, a high temperature flexural strength of 5.4MPa, and a residual flexural strength of 2.1MPa. For large parts with dimensions less than 800mm, a configuration of 2 surface layers, 1 transition layer, 6 reinforcing layers, and 1 sealing layer is used, resulting in a room temperature flexural strength of 3.8MPa, a high temperature flexural strength of 5.4MPa, and a residual flexural strength of 2.1MPa. For extra-large parts with dimensions between 800mm and 1300mm, a configuration of 2 surface layers, 1 transition layer, 8 reinforcing layers, and 1 sealing layer is used, resulting in a room temperature flexural strength of 3.7MPa, a high temperature flexural strength of 5.2MPa, and a residual flexural strength of 2.2MPa.

[0072] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element.

[0073] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0074] The above description of the disclosed embodiments enables those skilled in the art to make or use this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0075] Furthermore, it should be noted that the scope of the methods and apparatus in the embodiments of this application is not limited to performing functions in the order shown or discussed, but may also include performing functions substantially simultaneously or in the reverse order, depending on the functions involved. For example, the described methods may be performed in a different order than described, and various steps may be added, omitted, or combined. In addition, features described with reference to certain examples may be combined in other examples.

Claims

1. A low-cost method for preparing titanium alloy investment casting mold shells, characterized in that, Includes the following steps: Step 1: Prepare a surface layer and a back layer on the surface of the wax model; the surface layer is coated with a paint containing silica sol and zircon powder and sprinkled with zircon sand aggregate; the back layer is coated with a paint containing aluminum sol and mullite powder. Step 2: Dewax the mold shell to obtain a dewaxed mold shell; Step 3: Place the dewaxed mold shell in a vacuum calcination furnace. After evacuation, fill the furnace with hydrogen to form a low positive pressure hydrogen atmosphere. At the same time, continuously pump out the gas to remove the reaction products. Raise the temperature to 1100-1300℃ and keep it at that temperature. This causes the silicon-containing material in the surface layer to undergo a reduction reaction to generate gaseous products, which are then removed. This allows the surface layer to be transformed in situ into a ceramic layer with ZrO2 as the main phase. Step 4: Use the baked shell for titanium alloy casting.

2. The method for preparing a low-cost titanium alloy investment casting mold shell according to claim 1, characterized in that, In step 3, the furnace is first evacuated to below 1 Pa and then filled with hydrogen gas, maintaining the hydrogen pressure in the furnace at 0.05-0.15 MPa; the temperature is raised to 1100-1200℃ and held for 1-3 hours; then the temperature is lowered to 300℃, and the furnace is evacuated and the air is broken.

3. The method for preparing a low-cost titanium alloy investment casting mold shell according to claim 1, characterized in that, In step 1, the back layer includes a transition layer and a reinforcing layer sequentially disposed on the outside of the surface layer; The back layer also includes a sealing layer located outside the reinforcing layer. The transition layer, reinforcing layer and sealing layer are all coated with a coating containing aluminum sol and mullite powder. The number of the surface layer, transition layer, reinforcing layer and sealing layer is adjusted according to the size and structural complexity of the casting.

4. The method for preparing a low-cost titanium alloy investment casting mold shell according to claim 1, characterized in that, The dewaxing in step 2 is steam dewaxing, with a steam pressure of 0.75±0.05MPa, a temperature of 168±5℃, a pressure of 0.6MPa in 10 seconds, and a dewaxing time of 15-30min.

5. The method for preparing a low-cost titanium alloy investment casting mold shell according to claim 1, characterized in that, The silicon-containing material mentioned in step 3 includes SiO2 and ZrSiO4. The reduction reaction generates gaseous SiO and H2O, which are removed by continuous evacuation. Furthermore, the Al2O3 and mullite in the back layer do not undergo reduction reaction under the low positive pressure hydrogen atmosphere and continuous evacuation conditions.

6. The method for preparing a low-cost titanium alloy investment casting shell according to claim 1, characterized in that, In step 1, the coating temperature of the topcoat is 22±2℃, the humidity is 40-70%, and the drying time is 6-12h; the coating humidity of the backcoat is 30-60%, and the drying time is 12-24h.

7. A coating system for the preparation method of low-cost titanium alloy investment casting shells according to any one of claims 1-6, characterized in that, Includes topcoat and backcoat, wherein: The surface coating contains silica sol and zircon powder, which are used to undergo a reduction reaction under hydrogen vacuum calcination conditions and be transformed in situ into a ceramic layer with ZrO2 as the main phase. The back coating comprises aluminum sol and mullite powder, used to maintain structural stability under the hydrogen vacuum calcination conditions.

8. The coating system according to claim 7, characterized in that, The topcoat comprises, by weight, 10 parts silica sol, 38-42 parts zircon powder, 16-20 ml wetting agent, and 12-16 ml defoamer, with a flow cup viscosity of 22-32 s; the backcoat comprises a transition layer coating and a reinforcing layer coating. The transition layer coating comprises, by weight, 10 parts alumina sol and 15-20 parts mullite powder, with a flow cup viscosity of 12-16 s; the reinforcing layer coating comprises, by weight, 10 parts alumina sol and 28-32 parts mullite powder, with a flow cup viscosity of 23-28 s; and a sealing coating comprises, by weight, 10 parts alumina sol and 10-18 parts mullite powder, with a flow cup viscosity of 7-15 s.

9. A titanium alloy investment casting mold shell, having a face layer and a back layer, characterized in that, The surface layer is a ceramic layer formed by vacuum calcination and reduction with hydrogen, in which ZrO2 is the main crystalline phase and has a ZrO2 mass fraction of 80-95%; the back layer is a ceramic support layer composed of alumina sol as binder and mullite as refractory aggregate.

10. The shell according to claim 9, characterized in that, The surface layer contains less than 5% SiO2 by mass and less than 2wt% Si by element; the back layer contains 30-50% Al2O3 by mass.

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